Prior to a surgical intervention in respect of the eye of a patient—for example, the implantation of an intraocular lens—it is customary at the current time to register, by means of a suitable diagnostic instrument, biometric data pertaining to the eye, such as, for example, the depth of the anterior chamber, the thickness of the lens, the length of the eye and/or the refractive power of the cornea. On the basis of the registered refractive power of the cornea, the axial location of an astigmatism can be calculated. Based on the data acquired within the scope of the preliminary examination, markers are placed onto the eye, in particular onto the limbus, which during the intervention serve as a positioning aid for the surgeon, for example when introducing incisions or when inserting the intraocular lens into the eye.